From CGRP to PACAP, VIP, and Beyond: Unraveling the Next Chapters in Migraine Treatment.

Migraine is a neurovascular disorder that can be debilitating for individuals and society. Current research focuses on finding effective analgesics and management strategies for migraines by targeting specific receptors and neuropeptides. Nonetheless, newly approved calcitonin gene-related peptide (CGRP) monoclonal antibodies (mAbs) have a 50% responder rate ranging from 27 to 71.0%, whereas CGRP receptor inhibitors have a 50% responder rate ranging from 56 to 71%. To address the need for novel therapeutic targets, researchers are exploring the potential of another secretin family peptide, pituitary adenylate cyclase-activating polypeptide (PACAP), as a ground-breaking treatment avenue for migraine. Preclinical models have revealed how PACAP affects the trigeminal system, which is implicated in headache disorders. Clinical studies have demonstrated the significance of PACAP in migraine pathophysiology; however, a few clinical trials remain inconclusive: the pituitary adenylate cyclase-activating peptide 1 receptor mAb, AMG 301 showed no benefit for migraine prevention, while the PACAP ligand mAb, Lu AG09222 significantly reduced the number of monthly migraine days over placebo in a phase 2 clinical trial. Meanwhile, another secretin family peptide vasoactive intestinal peptide (VIP) is gaining interest as a potential new target. In light of recent advances in PACAP research, we emphasize the potential of PACAP as a promising target for migraine treatment, highlighting the significance of exploring PACAP as a member of the antimigraine armamentarium, especially for patients who do not respond to or contraindicated to anti-CGRP therapies. By updating our knowledge of PACAP and its unique contribution to migraine pathophysiology, we can pave the way for reinforcing PACAP and other secretin peptides, including VIP, as a novel treatment option for migraines.

Substance P inhibits bicarbonate secretion from guinea pig pancreatic ducts by modulating an anion exchanger.

The stimulatory pathways controlling HCO3- secretion by the pancreatic ductal epithelium are well described. However, only a few data are available concerning inhibitory mechanisms, which may play an important role in the physiological control of the pancreas. The aim of this study was to investigate the cellular mechanism by which substance P (SP) inhibits pancreatic ductal HCO3- secretion. Small intra/interlobular ducts were isolated from the pancreas of guinea pigs. During overnight culture the ducts seal to form a closed sac. Transmembrane HCO3- fluxes were calculated from changes in intracellular pH (measured using the pH-sensitive dye BCECF) and the buffering capacity of the cells. We found that secretin can stimulate HCO3- secretion in guinea pig pancreatic ducts about fivefold and that this effect could be totally blocked by SP. The inhibitory effect of SP was relieved by spantide, an SP receptor antagonist. SP had no effect on the activity of basolateral Na+-HCO3- cotransporters and Na+/H+ exchangers. However, the peptide did inhibit a Cl--dependent HCO3- efflux (secretory) mechanism, most probably the Cl-/HCO3 exchanger on the apical membrane of the duct cell.

Silencing of secretin receptor function by dimerization with a misspliced variant secretin receptor in ductal pancreatic adenocarcinoma.

Secretin receptors that are key for regulation of healthy pancreatic ductal epithelial cells have been reported to be functionally absent on ductal pancreatic adenocarcinomas. Here, we examine the possible presence and function of molecular forms of the secretin receptor in pancreatic cancer cell lines and in primary tumors. Surprisingly, reverse transcription-PCR and sequencing demonstrated wild-type secretin receptor mRNA in each of four cell lines and three primary tumors. Lack of biological response to nanomolar concentrations of secretin was best explained by the demonstrated coexpression of a second and predominant transcript in each of the cell lines and tumors. This represented a variant of the secretin receptor in which the third exon was spliced out to eliminate residues 44-79 from the NH(2)-terminal tail. This spliceoform has only recently been recognized in a rare gastrinoma, where it was incapable of binding secretin or signaling, and possessed dominant-negative activity to suppress hormone action at the wild-type secretin receptor (1). Overexpression of wild-type secretin receptor in Panc-1 cells driven by transfection of fully processed cDNA resulted in normal responsiveness to low concentrations of secretin, establishing the ability of these cells to produce a receptor capable of normal biosynthesis, trafficking, and signaling. Bioluminescence resonance energy transfer demonstrated that the variant receptor could form a heterodimer with wild-type receptor, providing a molecular mechanism for its dominant-negative activity. This suggests that missplicing is responsible for expression of a secretin receptor variant having the ability to suppress the function of wild-type receptor by a direct interaction. In 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assays in receptor-bearing Chinese hamster ovary cells, the secretin receptor was shown to have growth-inhibitory effects. Suppression of this activity in pancreatic carcinoma might, therefore, facilitate tumor growth and progression of this aggressive neoplasm.

Endothelin-1 inhibits secretin-stimulated ductal secretion by interacting with ETA receptors on large cholangiocytes.

We studied the expression of endothelin-1 (ET-1) receptors (ETA and ETB) and the effects of ET-1 on cholangiocyte secretion. The effects of ET-1 on cholangiocyte secretion were assessed in normal and bile duct-ligated (BDL) rats by measuring 1) basal and secretin-induced choleresis in vivo, 2) secretin receptor gene expression and cAMP levels in small and large cholangiocytes, and 3) luminal expansion in response to secretin in intrahepatic bile duct units (IBDU). ETA and ETB receptors were expressed by small and large cholangiocytes. ET-1 had no effect on basal bile flow or bicarbonate secretion in normal or BDL rats but decreased secretin-induced bicarbonate-rich choleresis in BDL rats. ET-1 decreased secretin receptor gene expression and secretin-stimulated cAMP synthesis in large cholangiocytes and secretin-induced luminal expansion in IBDU from normal or BDL rats. The inhibitory effects of ET-1 on secretin-induced cAMP synthesis and luminal duct expansion were blocked by specific inhibitors of the ETA (BQ-610) receptor. ET-1 inhibits secretin-induced ductal secretion by decreasing secretin receptor and cAMP synthesis, two important determinants of ductal secretion.

Gastrin inhibits secretin-induced ductal secretion by interaction with specific receptors on rat cholangiocytes.

We assessed the effect of gastrin on ductal secretion in normal and bile duct-ligated (BDL) rats. The effect of gastrin on ductal secretion was examined in the presence of proglumide, a specific antagonist for gastrin receptor (GR). We isolated pure cholangiocytes from normal and BDL rats and assessed gastrin effects on secretin receptor (SR) gene expression and intracellular adenosine 3',5'-cyclic monophosphate (cAMP) levels. We examined the presence of GR mRNA in cholangiocytes by reverse transcription polymerase chain reaction (RT-PCR). In normal or BDL rats, gastrin produced no changes in spontaneous bile secretion. Simultaneous infusion of gastrin inhibited secretin-induced choleresis and bicarbonate output in BDL rats. In the presence of proglumide gastrin did not inhibit secretin-induced choleresis in BDL rats. Gastrin decreased in cholangiocytes from BDL rats 1) SR gene expression and 2) secretin-induced cAMP levels. With the use of RT-PCR, GR mRNA was detected in cholangiocytes. Similar to what is shown for secretin and somatostatin, we propose that the opposing effects of secretin and gastrin on cholangiocyte secretory activity regulate ductal secretion in rats.

Interactions between secretin and acetylcholine in the regulation of fluid secretion by isolated rat pancreatic ducts.

1. Interlobular ducts were isolated from the rat pancreas and maintained in short-term tissue culture. Fluid secretion from these isolated ducts was measured using micropuncture techniques, intracellular calcium concentration ([Ca2+]i) by fura-2 microspectrofluorimetry, and cyclic AMP by radioimmunoassay. 2. Applying secretin and ACh simultaneously to ducts caused either a stimulation or an inhibition of fluid secretion depending on the doses employed. 3. The inhibitory effect of secretin and ACh could be relieved by atropine, and by the protein kinase C (PKC) inhibitors staurosporine and 1-(5-isoquinolinylsulphonyl)-2-methyl-piperazine (H-7). 4. Activation of PKC by 12-O-tetradecanoylphorbol-13-acetate (TPA) and phorbol 12, 13-dibutyrate (PDBu) inhibited secretin-evoked fluid secretion. 5. ACh and TPA also inhibited fluid secretion stimulated by the adenylate cyclase activator, forskolin. 6. Neither secretin nor the PKC activators and inhibitors had any effect on either the increase in [Ca2+]i evoked by ACh or the increase in intracellular cyclic AMP evoked by secretin and forskolin. 7. We conclude that the inhibitory effect of combined doses of secretin and ACh on ductal fluid secretion is probably mediated by PKC at a point in the secretory mechanism distal to the generation of intracellular messengers.

Somatostatin inhibits secretin-induced ductal hypercholeresis and exocytosis by cholangiocytes.

Previous work from our laboratory has implicated hormone-induced plasma membrane movement (i.e., endo- and exocytosis) in water and electrolyte transport by the epithelial cells that line the ducts in the liver (i.e., cholangiocytes). To further explore the cellular mechanisms regulating ductal bile secretion, we infused somatostatin and/or secretin intravenously into rats 2 wk after either bile duct ligation (BDL), a procedure that induces selective proliferation of cholangiocytes, or sham surgery and measured bile flow and biliary constituents. We also determined the effect of somatostatin on basal and secretin-induced exocytosis by purified cholangiocytes isolated from rat liver after BDL. Finally, we studied the expression of the somatostatin receptor gene by both ribonuclease (RNase) protection and nuclear run-on assays using cDNA encoding for two subtypes of the somatostatin receptor gene (i.e., SSTR1 and SSTR2). In vivo, somatostatin infusion caused a dose-dependent bicarbonate-poor decrease (57% maximal decrease below baseline; P < 0.05) in bile flow in BDL but not in sham-operated rats; in contrast, secretin caused a dose-dependent bicarbonate-rich choleresis (228% maximal increase above baseline; P < 0.05) in BDL but not in sham-operated rats. Simultaneous or prior infusion of somatostatin inhibited the secretin-induced hypercholeresis in BDL rats. In vitro, somatostatin had no effect on basal exocytosis by cholangiocytes isolated from BDL rats; however, somatostatin inhitibed (88% maximal inhibition; P < 0.05) secretin-induced exocytosis by cholangiocytes in a dose-dependent fashion. In addition, somatostatin inhibited secretin-induced increases in levels of adenosine 3',5'-cyclic monophosphate (cAMP) in cholangiocytes isolated from BDL rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Protective effects of therapy with a protease and xanthine oxidase inhibitor in short form pancreatic biliary obstruction and ischemia in rats.

The current study was done to evaluate the effects of short term (60 minutes) pancreatic biliary duct obstruction (PBDO) with intraductal hypertension (IDH) stimulated by secretin (0.2 clinical unit per kilogram per hour) and caerulein (0.2 microgram per kilogram per hour) plus 30 minutes of temporary pancreatic ischemia (ISCH) produced by ligation of celiac and superior mesenteric artery on the exocrine pancreas and protective effects of a new potent protease inhibitor, ONO3307 in combination with xanthine oxidase inhibitor, allopurinol, in this multifactor related model of acute pancreatitis in rats. Twelve hours after PBDO with IDH plus ISCH, we observed hyperamylasemia (23 +/- 3 units per milliliter) (p < 0.01); moderate pancreatic histologic changes; pancreatic edema (water content--81 +/- 2 percent) (p < 0.02), as well as the impaired amylase (2,889 +/- 328 units per kilogram per hour) (p < 0.01) and cathepsin B output (7 +/- 3 units per kilogram per hour) (p < 0.01) into the pancreatic juice of rats stimulated by caerulein (control group--serum amylase levels, 6 +/- 1 units per milliliter; pancreatic water content, 74 +/- 1 percent. Furthermore, PBDO with IDH plus ISCH caused the redistribution of lysosomal enzyme from lysosomal fraction (12 kilo times gravity pellet; 40 +/- 3 percent; p < 0.01) to zymogen fraction (1.3 kilo times gravity pellet; 38 +/- 3 percent; p < 0.01) (control group--12 kilo times gravity pellet, 59 +/- 2 percent; 1.3 kilo times gravity pellet, 24 +/- 2 percent) and the impaired pancreatic adenylate energy metabolism (0.79 +/- 0.02, p < 0.02) (control group--energy charge equals 0.88 +/- 0.01). Only PBDO with IDH caused no significant changes. Although only ONO3307 or allopurinol therapy showed the partial significant protective effects against pancreatic injuries, improving serum amylase levels, the administration of ONO3307 in combination therapy with allopurinol showed almost complete protective effects against the pancreatic injuries induced by PBDO with IDH plus ISCH (serum amylase levels, 9 +/- 2 units per milliliter; pancreatic water content, 76 +/- 2 percent; amylase and cathepsin B output, 7,127 +/- 946 and 18 +/- 3 units per kilogram per hour; 1.3 kilo times gravity pellet, 28 +/- 2 percent; 12 kilo times gravity pellet, 54 +/- 2 percent, and energy charge equals 0.85 +/- 0.02).(ABSTRACT TRUNCATED AT 400 WORDS)

In vitro inhibitory effect of somatostatin on secretin action in exocrine pancreas of rats.

BACKGROUND: Exocrine pancreatic function is influenced by pancreatic islet hormones. Although the existence of somatostatin receptors has been shown on pancreatic acinar cells, the in vitro effect of somatostatin on exocrine secretory function has not been established. METHODS: Using isolated rat pancreatic acini, the effect of somatostatin analog SMS 201-995 (SMS) and somatostatin 14 (S-14) on amylase release, cyclic adenosine monophosphate (cAMP) production, and hormone binding were determined. RESULTS: SMS inhibited the potentiating effect of secretin on amylase response to cholecystokinin octapeptide (CCK-8) in a concentration-dependent manner. The inhibitory effects of SMS and S-14 were similar on a molar basis and were observed when vasoactive intestinal polypeptide (VIP) but not 8bromoadenosine 3':5' cyclic monophosphate was used instead of secretin and when carbachol, bombesin, A23187, and 12-O-tetradecanoylphorbol 13-acetate were used instead of CCK-8. SMS inhibited secretin-induced cAMP production, and the dose-inhibition curve for cAMP was similar to that for amylase release. SMS had no influence on 125I-secretin and 125I-VIP binding. CONCLUSIONS: Somatostatin acts directly on acinar cells and inhibits secretin potentiation of secretory response in part by inhibiting secretin-induced cAMP production.

[Inhibitory effect of somatostatin analog, SMS 201-995, on exocrine secretion from isolated rat pancreatic acini].

In vitro effect of somatostatin analog, SMS 201-995 (SMS), on pancreatic exocrine secretion was investigated using isolated rat pancreatic acini. SMS had no effect on basal, cholecystokinin octapeptide (CCK-8)- or secretin-stimulated amylase release. SMS inhibited pancreatic amylase release in response to simultaneous stimulation with secretin and CCK-8 in a dose-dependent manner. Significant inhibition was observed with 10 nM SMS and maximal inhibition with 0.1-1 microM SMS. Amylase release in response to the combination of 100 pM CCK-8, 1 nM secretin and 0.1-1 microM SMS was similar to that to 100 pM CCK-8 alone. Secretin significantly increased acinar cell cAMP content. SMS partially inhibited an increase in cAMP content induced by secretin. The present study has demonstrated, therefore, that SMS directly inhibits the potentiating effect of secretin on exocrine secretion in part by inhibiting an increase in secretin-induced cAMP accumulation in rat pancreatic acinar cells.

Functional and structural characterization of the secretin receptors in rat gastric glands: desensitization and glycoprotein nature.

We have documented and characterized the down-regulation of the 125I-secretin binding sites and the associated desensitization of the secretin receptor-cAMP system in rat gastric glands. Secretin induced a rapid decrease of the high-affinity 125I-secretin binding sites with t1/2 = 30 min at 37 degrees C. Half-maximal down-regulation and desensitization occurred at 10(-9) M secretin, a physiological concentration corresponding to the half-maximal activation of the secretin receptor. The Scatchard parameters of the low-affinity 125I-secretin binding sites were unaffected by the pretreatment. This desensitization is heterologous in view of the loss of responsiveness to the truncated glucagon-like peptide 1 (TGLP-1), and pharmacologically selective since the secretin-related analogue VIP (10(-7) M) does not alter the secretin-induced cAMP generation in rat gastric glands. The glycoprotein nature of the secretin receptor has also been demonstrated using WGA-agarose affinity chromatography of the solubilized 125I-secretin receptor complex.

Effect of propranolol on secretin-induced gastrin release and secretin-induced tachycardia in patients with the Zollinger-Ellison syndrome.

The mechanism for secretin-induced gastrin release in the Zollinger-Ellison syndrome is uncertain. We evaluated whether the stimulatory effect of intravenous secretin on gastrin release was partly mediated through a beta-adrenergic stimulatory mechanism. Serum gastrin concentrations and heart rate were monitored in six patients with the Zollinger-Ellison syndrome. Secretin (2 clinical units/kg) increased mean serum gastrin concentrations from 1558 pg/ml basally to a peak of 3683 pg/ml (136% above baseline). This increase was not altered by pretreatment with 2 mg of propranolol intravenously, a dose which in previous studies blocked terbutaline-induced gastrin release. Secretin increased heart rate by 14 beats/min (20% above base-line) and this also was not altered by propranolol pretreatment. Thus, the stimulatory effects of secretin on gastrinoma cells and the heart do not appear to be mediated by beta-adrenergic receptors.

Role of secretin and CCK in the stimulation of pancreatic secretion in conscious dogs. Effects of atropine and somatostatin.

The role of gut hormones, such as secretin and CCK, in the stimulation of pancreatic secretion by duodenal HCl or oleate and by meat feeding has been studied in conscious dogs before and after pretreatment with atropine and somatostatin. Plasma hormones were measured by specific and sensitive radioimmunoassays. Duodenal perfusion with HCl and oleate stimulated dose-dependently pancreatic HCO3 and protein secretion and raised plasma levels of secretin and CCK, respectively. Atropine reduced significantly both HCO3 and protein secretion but did not affect plasma secretin and CCK levels in these studies. Both exocrine pancreatic secretion and plasma secretin and CCK levels were suppressed by somatostatin. Meat feeding caused a marked pancreatic HCO3 and protein secretion accompanied by a significant increase in plasma secretin and CCK which seem to play an important role in the postprandial pancreatic stimulation. Both atropine and somatostatin reduced the pancreatic secretion induced by exogenous hormones but only somatostatin, but not atropine, significantly decreased plasma secretin and CCK responses to intestinal stimulants. We conclude that both atropine and somatostatin reduce the pancreatic responses to duodenal HCl or oleate or to meat feeding but only somatostatin is capable of suppressing the release of secretin and CCK.

Peptide YY: metabolism and effect on pancreatic secretion in dogs.

Peptide YY is an ileocolonic peptide that inhibits meal-stimulated pancreatic secretion when infused in a dose of 400 pmol/kg X h. In this study pancreatic secretion was monitored in response to increasing doses of secretin or cholecystokinin-octapeptide (62.5, 125, 250, and 500 ng/kg X h) during the simultaneous infusion of either saline or peptide YY (400 pmol/kg X h). Peptide YY significantly (p less than 0.05) inhibited the secretory response to the three lowest doses of each pancreatic secretogogue, reducing the bicarbonate response to the 62.5-ng/kg X h dose of secretin by 86% +/- 6% and the protein response to the same dose of cholecystokinin by 57% +/- 16%. In the second limb of the study, the half-life of peptide YY (11.7 +/- 21 min) and the metabolic clearance rate (13.8 +/- 1.6 ml/kg X min) were found to be similar to those of other gastrointestinal hormones. We conclude that inhibition of meal-stimulated pancreatic secretion by peptide YY can be explained by its ability to decrease the responsiveness of the pancreas to endogenous secretogogues.

Somatostatin analogs: correlation of receptor affinity with inhibition of cyclic AMP formation in pancreatic acinar cells.

Somatostatin inhibited secretin-stimulated cyclic AMP formation in pancreatic acinar cells. The inhibition was only partial. Maximal inhibition reached about 50%. Somatostatin analogs tested inhibited secretin-stimulated cyclic AMP formation with a lower potency than somatostatin. Cys-Aza Ala-Phe-Phe-DTrp-Lys-Thr-Phe-Phe-Cys was found to be an antagonist of somatostatin in inhibiting secretin-stimulated cyclic AMP. Analogs inhibited the binding of 125I-[Tyr11] somatostatin to pancreatic acini. There was a good correlation (r = 0.97) between concentration for inhibiting 50% secretin-stimulated cyclic AMP and receptor binding affinities.

Effects of dopamine on exocrine secretion and cyclic nucleotide concentration in the dog pancreas.

The effects of intravenous injection of dopamine on pancreatic exocrine secretion and on pancreatic cyclic AMP and cyclic GMP concentrations of mongrel dogs were compared with the effects of secretin and pancreozymin. Dopamine (1--10 micrograms/kg), secretin (0.03--0.3 units/kg) and pancreozymin (0.1--1 units/kg) increased exocrine secretion dose dependently, Sulpiride (0.3--1 mg/kg) and yohimbine (0.3--1 mg/kg), D2-receptor antagonists, inhibited the dopamine-induced exocrine secretion dose-dependently but did not inhibit the secretin- or pancreozymin-induced secretion. Secretin (0.3 units/kg) increased cyclic AMp concentration by about 50% but did not affect cyclic GMP concentration. Pancreozymin (1 unit/kg) slightly increased cyclic AMp concentration but markedly increased cyclic GMP concentration by about 50%. However, dopamine (10 micrograms/kg) increased neither cyclic AMP nor cyclic GMP concentration. These results suggest that dopamine causes exocrine secretion from the dog pancreas through D2-receptors which are not linked to adenylate cyclase.

Effect of somatostatin on determinants of bile flow in unanesthetized dogs.

Seven dogs each underwent cholecystectomy, ligation of the accessory pancreatic duct, and insertion of a Thomas duodenal cannula opposite the ampulla of Vater. After full recovery, bile secretions were studied in the unanesthetized dogs by opening the cannula and placing a ureteric catheter through the papilla into the common bile duct. All animals received, throughout study, constant infusions of taurocholic acid to replace losses caused by interruption of the enterohepatic circulation and 14 C-erythritol for measurement of erythritol clearance. After bile flow stabilized somatostatin 800 ng/kg/minute was infused for 100 minutes and bile flow declined from 3.0 +/- 0.7 ml/10 minutes (SD) to 1.19 +/- 0.47 ml/10 minutes (p less than 0.001) and 14C-erythritol clearance fell from 3.6 +/- 1.14 to 1.77 +/- 0.43 ml/10 minutes (p less than 0.001). Bile salt output was unchanged, indicating that somatostatin inhibited bile salt-independent canalicular flow (BSICF). In other experiments animals underwent intraduodenal acidification which resulted in a marked increase in bile flow. Somatostatin infusion again causes a sharp fall in bile flow (p less than 0.05) suggesting that somatostatin also inhibited ductular flow. Infusion of somatostatin did not inhibit choleresis produced by exogenous secretin administration. Thus, somatostatin inhibits 1) ductular flow by inhibiting secretin release and 2) BSICF by a direct effect or by decreasing the release of hormones which induce canalicular flow.

Effect of extracellular calcium on amylase release from dispersed pancreatic acini.

In dispersed acini prepared from guinea pig pancreas, removing extracellular calcium did not alter the basal rate of amylase release but reduced the stimulation of enzyme release caused by cholecystokinin, carbachol, secretin, and vasoactive intestinal peptide as well as that caused by derivatives of cyclic nucleotides. In acini incubated in a calcium-free, EGTA-containing medium the increase in amylase release caused by each secretagogue tested did not change during the initial 10 min of incubation, decreased by approximately 65% during the subsequent 40 min, and remained constant thereafter. Removing extracellular calcium did not alter the maximally effective concentrations of cholecystokinin or vasoactive intestinal peptide but abolished the decrease in stimulated enzyme secretion seen with supramaximal concentrations of cholecystokinin. Incubating pancreatic acini with cholecystokinin or carbachol plus secretin or vasoactive intestinal peptide caused potentiation of amylase release, and removing extracellular calcium reduced the stimulation of enzyme release caused by the two secretagogues in combination but did not alter their potentiating interactions.

Inhibition of secretin stimulated pancreatic secretion by pancreatic polypeptide.

The effect of PP on secretin-stimulated pancreatic secretion was assessed in five healthy subjects. During an intravenous infusion of BPP at a dose which produced plasma levels similar to those seen after meals in healthy young adults the volume and bicarbonate content of duodenal juice was reduced by 25% (p less than 0.05) and 24% (p less than 0.05) respectively, while protein and bilirubin concentrations were more markedly reduced by 68% (p less than 0.0005) and 67% (p less than 0.0005) respectively. PP, thus, may be an important inhibitory factor in the control of bilirubin and pancreatic enzyme secretion in man.